Method, device and use of said method for biological elimination of metal elements present in an ionized state in water

ABSTRACT

The method, in which the water to be treated is partly oxygenated by a specific aeration carried out before percolation through a biofilter having a bacteria-supporting bed of filtering material, has a measurement stage of at least one parameter constituted by the oxidation-reduction potential of the aerated water before passage into the biofilter, a transmission stage of measurement signals to a computer and comparison of the signal representative of the value of the measured parameter with at least one lower limit set in function of the measurement carried out in a second stage for measuring a parameter representative of the pH and a stage for correcting the air flow by a signal determined by the computer in function of the two preceding stages.

The present invention relates to a method for optimizing and monitoringautomatically, by biochemical route, the elimination parameters ofmetallic elements present in the ionized state in water, for exampleground or surface waters, the device for implementing said method anduse of said method.

More particularly, the invention concerns a method and a device forelimination by biological route of divalent elements, such as divalentiron and manganese, present in groundwater.

The invention can be extended to surface waters devoid of dissolvedoxygen where these elements are present in the same state, such as thereducing hypolimnion water above a dam in a state of eutrophication.

The oxidation of minerals by biological route has already been theobject of in-depth studies and practical applications. This family ofmethods uses the capacity of certain specific bacterial strains,indigenous and/or incorporated, to catalyse by the exothermic oxidationreactions by enzymatic conversion. In return, these exothermic oxidationreactions provide the bacteria with the energy necessary for theirdevelopment. This family of methods has been applied in particular inthe mining industry for many years.:

either in the domain of extractive hydrometallurgy, whose first phasesconsist of pulverisation of the ore, enrichment by flotation andleaching in an acid or alkaline medium; biological leaching, or“bioleaching”, often in competition now with purely chemical leaching.The most widespread applications have concerned, until recent years, thecopper industry (see the work by N. N. Hughes & R. K. Poole: “Metals &Micro-organisms”, published by Chapman & Hall, 1989, and the article byD. Morin: “Biotechnologies dans la métallurgie extractive”, published inLes Techniques de l'Ingénieur, Paris 1995, No. M2238, vol. 1): the ore,more or less fractionated, is simply heaped in the open air andsprinkled with a solution of nutritive elements. This method, knownunder the name of “heap leaching”, does not require any precisemonitoring during the operation;

or for the treatment of acid effluents containing high amounts ofdissolved divalent iron. The Japanese patent No. 44717/72 describes amethod in which the culture of iron bacteria was carried out in atreated effluent, and then tipped into the effluent to be treated. Inthe Japanese patent No. 38 981/72, the method was improved by producingthe bacterial culture ‘in situ’, on supports constituted of iron oxides.A further improvement was provided by the French patent No. 2.362.793,filed in 1976, where the bacterial culture was fixed on an insolublesupport, at the pH of the effluent, and where the ferrous irons (Fe²⁺)were oxidised by air blowing into an agitated reactor. The bacteria andtheir supports were then separated by decanting, then recycled in thereactor, as in a system for treating urban waste water by the activatedsludge method. In such cases as well, control of the process does notpose any problem. In particular, the low pH level of the medium avoidsany competition between the physico-chemical route and the biologicalroute for iron oxidation, which in practice means absence of the needfor monitoring the quantity of oxygen introduced and the quantity ofresidual oxygen in the water after treatment.

Later, it was considered that these phenomena could be applied to theelimination, by biological route, of the iron and manganese present inthe dissolved state in natural water deprived of oxygen and whose pHvalue, contrary to the effluents mentioned above, is situated close toneutrality, roughly plus or minus one unit. In this field, even if itconcerns species different from those characteristic of acid effluents,it was already known that bacteria capable of catalysing iron and/ormanganese oxidation, still called ferro- and mangano-bacteria, could,thanks to exogenous enzymes and/or polymers, by detected in very diverseenvironments (groundwater, lake beds, emergent springs in marine bays,etc . . . ). The damage from ferro- or mangano-bacteria clogging up welldrains or corroding metallic piping was already well known, thesebacteria needed to be “domesticated” to make them work usefully inplants for eliminating dissolved iron and manganese.

The first observations on this subject were published by U. Hässelbarth& D. Ludemann (in the article “Die Biologische Enteisenung undEntmanganung”, Vom Wasser, 1971, vol. 38, pp. 233-253; “Removal of ironand manganese from groundwater by microorganisms”, Water Treatment &Examination, 1973, vol. 22, No. 1, pp. 62-77 and were concretised by thesame authors filing the German patent No. 1.767.738. This patentdescribes a method for biological iron removal by oxygenation andfiltration, under conditions such that the power of oxidation-reductionof the medium has a value rH higher than or equal to 14.5±0.5. The rH isan index analogous to the pH, representing quantitatively the value ofthe oxidising or reducing power of a medium. This rH value correspondsto the lower limit of the domain of action of ferro-bacteria. Thus aminimum condition was defined, but it only represented a threshold,moreover insufficient, for total iron removal and it did not make itpossible to define the lower and upper limits on which to base theautomatic regulation of the method.

Besides, the authors defined very limited and very restrictiveoxygenation conditions for the method, probably because they did nothave a variety of types of untreated water available. The development ofthis promising method was thus delayed.

At about the same time, similar studies were undertaken in France (P.Mouchet & J. Magnin: “Un cas complexe de déferrisation d'une eausouterraine”, TSM-l'eau, 1979, vol. 74, No. 3, pp. 135-143), but forvery differing waters, which made it possible for the French researchersto define more precisely the limits for biological iron removal and toprovide, at the 1985 “Wasser Berlin” Congress, the construction of aboutthirty installations based on this principle of biological treatment(see the article of P. Mouchet et al. entitled “Elimination of iron andmanganese contained in groundwater: classic problems, recent progress”,published in Water Supply, 1985, vol. 3, No. 1, pp. 137-149). Besides,at the same time, the German researchers had noted that many plantscould operate spontaneously on this principle (see the article by C.CZEKALLA et al. entitled “Quantitative removal of iron and manganese bymicroorganisms in rapid sand filters (in situ investigations)”,published in Water Supply, 1985, vol. 3, No. 1, pp. 111-123). Moreover,similar observations had stimulated French research at the beginning ofthe seventies.

Thus from reading the above, one can note that biological iron removalwas the method studied the most and the best known at the beginning ofthis research, probably because the natural seeding by indigenousbacteria is relatively rapid. On the contrary, the seeding timeconcerning biological manganese removal, a matter of several weeks, oreven two or three months, did not encourage studies on manganeseremoval, at least to begin with.

The results of French research, published in 1985, described amongothers the field of activity of ferro-bacteria, such as shown in FIG. 4,within which, moreover, the domain of existence of a treatment ensuringtotal iron elimination is submitted to more restrictive limits.

Such a diagram, drawn with ordinate the oxidation reduction potentialand with abscissa the pH, is called a stability diagram. It was drawn upfirst by M. J. POURBAIX to study corrosion phenomena of ferrous metals,and was later extended to the speciation of the principal elements (seethe works of M. J. Pourbaix entitled “Atlas d'equilibresélectrochimiques à 25° C.”, published by Gauthier-Villars, Paris 1963,pp 307-321) and applied to the study of iron removal from groundwater(see article by J. D. Hem, entitled “Stability field diagrams as aids iniron chemistry studies”, published in the AWWA Journal, 1961, Vol. 53,No. 2, pp. 211-232).

The domain of biological iron removal (Db) or ferro-bacterial activityis defined by a minimum rH value and a maximum rH value, the zonecomprised below the curve (rHmin) representing the minimum rH valuecorresponding to the stability domain (DFe²⁺) of the ferrous ion and thezone comprised above the curve (rHmax) representing the maximum rHcorresponding to the domain (Dpc) of physico-chemical iron removal. Theoptimum domain for biological iron removal (Db) overlaps the theoreticallimit (DFe²⁺/Fe³⁺) separating the respective domains of the ferrous ironand the ferric hydroxide.

The diagram in FIG. 4 shows that biological iron removal from naturalwater, whose pH can vary from values lower than 6 up to values higherthan 8, can only be produced under certain conditions ofoxidation-reduction potential and pH for which there is enzymaticoxidation of the ferrous ions Fe²⁺ without precipitation of basic saltsof ferric iron Fe³⁺, that is without production of physico-chemical ironremoval whose performances are much more modest than those of biologicaliron removal.

In the water before treatment, the oxidation-reduction potential (or Eh)varies in function of the concentration of oxygen dissolved brought byaeration, whereas in treated water the Eh depends more on the value ofthe Fe³⁺/Fe²⁺ couple, that is to say the degree of iron oxidation. InFIG. 4, it can be clearly seen that the oxidation conditions must bemore closely monitored as the pH rises. In particular, when the pH isgreater than 7.6 in untreated water, the level of oxygen dissolved mustbe lower than a maximum threshold, very limited, the exact upper limitbeing lowered as the pH rises. This pH value is comprised between 7.6and 8.5 defining a domain (Dbc) where the iron removal is difficult toadjust because of the competition between the biological oxidation andthe physico-chemical oxidation.

Since a level of oxygen dissolved at a concentration higher than 50% ofsaturation point is nonetheless desirable in treated water, inparticular to avoid fermentation and corrosion during distribution,these observations have led to the concept of biological iron removalplants, for all waters at pH>7, following the diagram of FIG. 5. In thisdesign, untreated water undergoes a first aeration carried out in amixer 31, specially studied to obtain an immediate mixture of air andwater. The water is then submitted to biological iron removal byhigh-speed percolation through a bed of specific filtering material, aferro-bacteria support in a biofilter reactor 32 studied specially forthis. The filtered water is then submitted to a final intensive aerationin a new mixer 33. The filter bed is made of a material marketed underthe brand name Biolite®.

The necessity for perfect control of the quantity of oxygen injected forall waters with pH>7.3 led to the French patent No. 2. 470.094, filed in1979 by the Applicant. This patent describes an invention according towhich the oxygen was introduced into the water to be treated byrecycling part of the treated water. This treated water was previouslybrought close to saturation in dissolved oxygen through a finalintensive aeration, the quantity of oxygenated recycled water being afunction of the pH of the water to be treated and its oxygenrequirements, and adjusted using a rotameter or other apparatus formeasuring flow.

Continuing research in France then made it possible to set the limits ofthe domain for biological manganese removal and to compare them withthose of biological iron removal. FIG. 6 shows that the two domains areseparate and that there is no common point between the zone (A) of thebiological route for iron removal, overlapping the theoretical limit(DFe²⁺/Fe³⁺) separating the respective domains of the ferrous ion andferric hydroxide, and zone (B) of the biological route for manganeseremoval overlapping the theoretical limit (DMn²⁺/Mn⁴⁺) separating therespective domains of the Mn²⁺ ion and manganese dioxide (MnO₂).

The result is that for water containing the two elements together, thesolution generally adopted is treatment in two filtration stages,described in the article by P. Mouchet “From conventional to biologicalremoval of iron and manganese in France”, published in the Journal AWWA,1992, vol. 84, No. 4, pp. 158-167, a first for iron removal and a secondfor manganese removal, each stage specifically receiving its ownadjustments of oxygen injection and, if required, adjustment of the pH.

This know-how represents the prior state of the art, before the presentinvention, from which it is clear that for correct running of atreatment plant for iron removal and, if necessary, manganese removal bybiological route, the surrounding conditions in the treatment mediummust be adjusted continually, in order to avoid any inhibition ofbacterial activity. These conditions depend on a certain number ofparameters, such as the pH, the oxidation-reduction potential, thetemperature, the substrate concentrations, the element to be oxidised,and oxygen.

Any fault in adjustment or operation of the oxygen-supply and/or pHcorrection devices leads to malfunction of the treatment unit, resultingin a lowering of the oxidation efficiency of the element to be oxidised.Nonetheless, biological elimination has many advantages overconventional physico-chemical treatment:

higher quality of treated water,

compactness of treatment plants,

higher treatment speed,

absence of reactive agents (flocculants, oxidants),

loss of charges reduced,

significant reduction of investment and running costs,

better production yields of dehydration of sludge, enough significantadvantages to be a real incentive to overcome the defects andmalfunctions mentioned above.

In the domain of biological iron removal, said defects and malfunctionscan arise from:

either a lack of oxygen, which would limit the respiration needs of thebiomass, and would situate the water at a level of lowoxidation-reduction potential;

or an excess of oxygen, which would situate the water at a level of toohigh oxidation-reduction potential, and would inhibit microbe activity.These conditions could even create competition between physico-chemicaloxidation and bacterial activity;

or too low an acid pH (less than 6 to 6.5) which would situate the waterunder the minimum threshold for oxidation-reduction potential (Eh),required for micro-organisms to react to provide total oxidation of theelement to be eliminated;

or, finally, too high a basic pH (over 7.8 to 8) which would situate thewater above the maximum threshold for oxidation-reduction potential,encouraging competition between chemical oxidation of the element to beoxidised and bacterial activity, even going as far as inhibiting thisbacterial activity.

On the other hand, biological manganese removal is less sensitive to themedium parameters, although it is nevertheless useful to ensurepermanently that, before entering the manganese removal reactor, thewater to be treated has a sufficiently high pH (greater than about 7.2)and a content of dissolved oxygen greater than 50-60% saturation to bein the domain (B) defined in FIG. 6. The control of these biologicaltreatments therefore rests on that of the operational physico-chemicalparameters, which is not easy for small plants and when the oxygencontent (O₂) dissolved has to be very low for the process (sometimesless than 1.0 mg/l for biological iron removal from waters with pHhigher than 7.5). The present invention solves this type of problem byproviding a method for regulating the process in function of thecharacteristics of the raw water.

It can be noted that a similar philosophy for biological processes hasalso appeared in other domains, for example:

for anaerobic treatment of waste water (see patent FR 2 672 583 or U.S.Pat. No. 5,248,423, filed by the Applicant in 1992).

for extractive hydrometallurgy , for biofiltration of metals more rareand more precious than copper, for example gold, (see the article by A.Kontopoulos & M. Stefanakis, “Process options for refractory sulfidegold ores: technical, environmental and economical aspects”, 1991, 393;the article by J. Libaude, “Le traitement des minerais d'or” publishedin Recherche, May 1994) or cobalt (see the article by D. Morin et al.“Study of the bioleaching of a cobaltiferrous pyritic concentrate”,published in IBS proceedings, 1993, vol. 1, p. 147; the article by D.Morin “Des bactéries vont extraire le cobalt”, published in Recherche,1998, No. 312, pp. 38-40). It should be noted that the gangue, fromwhich these metals are extracted, is often constituted mainly of pyrite,which here again implies a major action of ferro-bacteria adapted toacid media, already mentioned above relative to the treatment of acideffluents from mines.

Nonetheless, the invention described here has no point in common withthe above domains, where for example the composition ofmethane-containing gas intervenes, on the flow of raw water underanaerobic treatment of effluent, or a process for bioleaching iscontrolled by reactor hydraulics, the introduction of adapted bacterialstrains and/or the adjustment of the temperature.

In the present case, the conditions of oxidation of raw water willprimarily be adjusted automatically as a function of the characteristicsof the raw water, the aerated water before treatment and/or the finaltreated water. A first approach to this type of regulation was tried outon the bases of the Eh potential of the water treated (see the articleby C. Tremblay et al., “Control of biological iron removal from drinkingwater using ORP”, published in IAWO, Vancouver, 1998, June) similar toother devices studied for treating waste water (see the article by J.Charpentier et al, “Oxidation-reduction potential (ORP) regulation: Away to optimize pollution removal and energy savings in the low loadactivated sludge process”, published in Water Sci. Tech., 1987, vol. 19,No. 3-4, pp. 645-656; and the article by D. G. Warcham et al, “Real-timecontrol of wastewater treatment systems using ORP”, published in Wat.Sci. Tech., 1993, vol. 28, No. 11-12, pp. 273-282). The tests based onthis principle, carried out by the Applicant, resulted in failure.

In fact, in treated waste water, the final oxidisation-reductionpotential Eh takes into account the transformation of carbonaceous,nitrogenous, phosphorated, sulphurated etc. species, of which only afraction can be eliminated from the water by stripping or stocking inbacteria: a fraction of these compounds therefore remains dissolved,under a partly reduced/partly oxidised form, and the final potentialdepends on the respective proportions of the two forms of theoxidation-reduction system.

On the other hand, as far as iron and manganese removal are concerned,the reduced forms of the metals are oxidised and precipitated, thusquasi-eliminated from the dissolved matrix. For a given pH, the finaloxidation-reduction potential Eh is representative of this elimination,whether it takes place by physico-chemical or biological route.Furthermore, it is independent of the content of dissolved oxygen, sincethe normal potential of the O₂/H₂O couple is very much lower than thatof the Fe³+/Fe²+ couple. Measurement of the Eh of treated water thus hasa certain interest as a parameter indicative of the efficiency of thetreatment, which has moreover been demonstrated by the authors quotedabove, who were able to establish a significant relationship betweenthis value and the residual iron content in the filtered water. On theother hand, in no case can this measurement serve as a basis forregulating the process.

The aim of the present invention is to propose a process making itpossible to avoid such risks and to regulate a treatment for biologicalelimination of elements present under ionized form.

This aim can be attained through the fact that the elimination process,by biological route, for metallic elements present in the ionized statein waters devoid of dissolved oxygen, in which the water to be treatedis partially oxidised by a specific aeration carried out beforepercolation through a biofilter reactor including a bacteria-supportingbed of filtering material, is characterised in that it comprises:

a measurement stage for at least one parameter constituted by theoxidation-reduction potential (Eh) of the aerated water before passageinto the biofilter;

a transmission stage of measurement signals to a computer and comparisonwith the signal representative of the value of at least one parametermeasured for at least one lower limit of this parameter determined infunction of the measurement carried out in a second stage of measurementof a second parameter representative of the pH and

a possible stage of correction of the air flow by control by a unit foradjusting the air flow by a signal determined by the computer infunction of the two preceding stages.

According to another characteristic, the process comprises a stage formeasuring the second parameter constituted by the pH of the aeratedwater before passing into the biofilter, a stage of comparison with alower and higher limit of the first parameter whose lower and higherlimits are determined in function of the measurement of the secondparameter.

According to another characteristic, the process comprises a stage forcompensation of a fault in the regulation of the air flow by the firstparameter by means of a system of complementary regulation using atleast one signal provided by a means of measurement of the residualcontent of oxygen dissolved in the treated water.

According to another characteristic, the compensation stage uses asecond signal provided by means of measurement of the pH of the treatedwater simultaneously with measurement of the dissolved oxygen.

According to another particularity, the process comprises a stage forregulation of the pH of the filtered water by injection of an alkalinesolution into the water to be treated; if the signal provided by meansof the measurement of the pH and representative of the value of the pHof the water treated is lower than a fixed lower value, said injectionbeing limited by a fixed higher set value of the pH.

According to another particularity, the process comprises a stage forverification of the efficiency of the treatment by continuousmeasurement of the residual dissolved iron content andoxidation-reduction potential of the filtered water, with an alarm beingset off in the event of an anomaly.

Another aim of the invention is to propose a device for elimination ofelements present under ionized form in ground water or surface water.

This aim is attained by the fact that the device for the treatment ofwater devoid of dissolved oxygen according to the invention ischaracterised in that it comprises an aeration chamber into which theraw water and injected air are introduced, whose flow is adjusted by avalve allowing controlled aeration under pressure and whose outlet islinked to a biofilter reactor, provided with an exit, with a porousferro-bacteria supporting bed through which the water to be treatedpercolates, first pH measurement means and second oxidation-reductionpotential measurement means of the aerated water, set between theentrance chamber and the filter, calculating units taking into accountthe signals delivered by the first and second measurement means todeliver a command signal to a means for adjusting the air flow acting onthe valve to enable regulation in function of an upper and lower limitof Eh potential, determined for a given pH value.

According to another characteristic, the device comprises pH measurementmeans and means for measuring the dissolved oxygen at the exit from thefilter and units for calculating and regulating the air flow to allowcomplementary regulation of the process.

According to another characteristic, the device comprises a pHregulation station comprising a reservoir of alkaline solutioncontrolled by an electro-valve or a feed pump which is monitored by thesignal worked out by a regulation unit and allowing regulation of the pHin function of the signal delivered by the pH measurement means set atthe exit from the biofilter reactor to the regulation unit.

According to another particularity, the device comprises means formeasuring the oxidation-reduction potential of residual iron in thefiltered water, set at the exit from the filter and enabling evaluationof the efficiency of the device.

According to another particularity, the exit from the biofilter reactorwith ferro-bacteria is linked to the entry of a second aeration chamberin which the treated water is sent from the biofilter reactor withferro-bacteria and air injected by a second valve, the exit from theaeration chamber being linked to a second biofilter reactor providedwith an exit and lined with a porous mangano-bacteria supporting bedthrough which the treated water coming from the ferro-bacteria biofilterfilters and third measurement means of oxidation-reduction potential atthe exit from the second aeration chamber, a computing unit taking intoaccount the signal delivered by the third measurement means to deliver acommand signal to a means of regulation acting on the second valve toallow regulation of the air flow in function of a given lower limit.

According to another characteristic, the filter bed is constituted ofsiliceous sand of effective size between 1 and 3 mm.

According to another characteristic, the filter bed is constituted of afiltering material, called “Biolite”, specially designed for this typeof treatment.

Another aim of the invention is to propose a utilisation for saidprocess.

This aim is attained in that the process is used in a biofilter with aferro-bacteria supporting bed of filtering material.

According to another characteristic, the process is used in a biofilterwith a mangano-bacteria supporting bed of filtering material.

According to another characteristic, the process is used in a biofilterwith an autotrophic bacteria supporting bed of filtering material.

According to another characteristic, the process is used in a firstbiofilter with a ferro-bacteria supporting bed of filtering material,and then the treated water exiting from this first biofilter is used ina second biofilter with a mangano-bacteria supporting bed of filteringmaterial.

Other characteristics and advantages of the present invention willbecome clear by reading the description below referring to the attacheddrawings in which:

FIG. 1 shows a diagram of a device, according to the invention, for ironremoval from ground water;

FIG. 2 shows an iron removal and manganese removal device for wateraccording to the invention;

FIG. 3 shows the results of biological iron removal according to theinvention compared with those obtained according to physico-chemicaliron removal;

FIG. 4 shows a stability diagram of prior art illustrating the domain ofactivity of ferro-bacteria;

FIG. 5 shows a device for biological iron removal of prior art;

FIG. 6 shows the biological iron removal and manganese removal domainsdefined by prior art.

The invention will now be described with reference to the figures.

The process consists of measuring the following parameters: the pH andoxidation-reduction potential, on previously aerated water, and possiblythe pH and/or dissolved oxygen on the water treated by the biomass. Fromthese measurements and in real time the monitoring unit acts ondifferent adjustment mechanisms in order to adjust the operationalconditions best adapted to the correct operation of the ecosystem in thereactor where the biological reaction takes place with the biomass.These control units are such as indicated in FIG. 1, which is only oneembodiment of a device enabling implementation of the process accordingto the invention, given as a non-limiting example and detailed asfollows.

The raw water, still called water to be treated, is carried by piping 1,to which an air injection piping 2 is connected. An automatic valve 3regulates the flow of the latter; the water is immediately mixedthoroughly with the introduced air, passing into an aeration chamber ormixer 4, and then penetrates into a biofilter reactor 5, filled with aspecific filtering material 6, called a filter bed, resting on aflooring 50 provided with a plurality of nozzles 51. The filter bed canbe constituted of siliceous sand of effective size between 1 and 3millimetres, or of porous material designed specially for biofiltration,of the type on sale under the brand name “Biolite”. After treatment, theeffluent exits from the biofilter 5 through the piping 7 for treatedwater, connected below the flooring 50. On the raw water piping 1,downstream from the mixer 4, a sensor-analyser assembly 8 constitutes afirst measurement means of the pH of the aerated water, whereas ananalogous apparatus 9 constitutes a second means for measuring theoxidation-reduction potential (Eh) of the aerated water. The signalsrepresenting the results of the two analyses are transmitted to acomputer 10 which checks that the value of the Eh potential is reallybetween a minimum (lower limit) and a maximum (upper limit) determinedin function of the pH value of raw water. If this is not the case, thecomputer 10 sends a signal to a regulator 11 which provides the order toincrease or reduce the air flow delivered by the valve 3 depending onwhether the value of the Eh potential of the aerated water is below thelower limit or above the upper limit respectively.

Thus, as described above, the specific filtering material is constitutedeither of sand, or of a “Biolite” type material of effective size higherthan the order of 1 to 3 mm, such as for example 1.25 mm, that isgreater by 50% than the effective size of 0.95 to 0.75 mm of the samefilters used under conditions not corresponding to iron removal andmanganese removal conditions. In the same way, the nozzles 51 of theflooring of the biofilter reactors can comprise wider slits of the orderof 0.7 to 1.2 mm whereas formerly, the slits had a size of 0.4 mm. Thefiltration speed is of the order of 30 to 50 m/sec.

Finally, the oxygenation created by the regulation will generate thedevelopment of bacteria within the filter, these bacteria beingferro-bacteria or mangano-bacteria according to the specific aerationconditions created upstream of the filter. The effective size of thefiltering bed and the nozzle slits makes it possible, taking intoaccount the lower size of the bacteria, to avoid obstruction of thefiltering bed and the nozzles and above all to wash the raw water filterbed, when the passage speed of water in the filter exceeds thefiltration speed.

A precision adjustment, complementary to the main adjustment of theprocess described above, is ensured by control units located downstreamfrom the biofilter 5. The exit 7 for filtered water is provided with ameasurement means 12 for residual dissolved oxygen and a secondmeasurement means 13 for the pH. The signals representative of themeasurements are sent to a calculation unit or a computer 14 whichverifies that the signal representative of the content of dissolvedoxygen, for the pH values measured, is neither below a given lowerthreshold, for the pH value measured, due to consumption of part of theoxygen introduced ahead during iron oxidation, nor above a given upperthreshold, resulting from a possible lack of precision of the regulationof air flow resulting from the Eh potential measurement of aeratedwater. To begin with, overrunning one of these thresholds detected bythe computer 14 can set off an alarm 15 which will alert the operator toverify the upstream regulation (through the Eh potential value ofaerated water) and if necessary to adjust the values of orders sent tothe computer. After this, if optimisation of upstream regulation of thebiofilter is impossible, this will be replaced by a downstreamregulation through dissolved oxygen, carried out by a signal emitted bythe computer 14 to a regulator 16 which sends, depending on the case, asignal to open or close the air entry valve 3 to return within the setlimits.

Furthermore, it must be remembered that the iron oxidation andprecipitation reactions freeing protons H⁺, are acidifying. If thebuffering capacity of the raw water is low, corresponding to lowalkalinity, during the process the pH risks undergoing a dropincompatible with good treatment yield. The device according theinvention comprises a pH regulation station making it possible to avoidthis drop in pH. In order to eliminate this disadvantage, the processaccording to the invention envisages sending the result of the pHmeasurement provided by the second measurement means 13 to a regulationunit 17 which starts, if the pH descends below a certain set point(lower threshold), the progressive operation of an electro-valve or afeed pump 18 which introduces into the piping 1 an alkaline solution 19contained in a preparation tank or reservoir 20, without the water pHgoing beyond the upper set limit (upper threshold), compared with the pHmeasurement provided by the measurement means 8 or the measurement means13. The lower and upper pH thresholds are decided in function of thenature of the bacteria used, each bacterium having a preferential pHrange.

The lower and upper limits attributed respectively to theoxidation-reduction potential Eh of the aerated water and to theconcentration in dissolved oxygen of the treated water are deduced bysimple algorithms whose independent variable is the pH of thecorresponding water and which are stored in the memory of the computerad hoc.

The algorithms used correspond to:

the upper and lower limits of the Eh oxidation-reduction potential ofthe aerated water, characterised by an equation of the form

Eh=α−β*pH

the upper and lower limits of the oxygen concentration [O₂] of thetreated water (for a pH equal to or greater than 7), characterised by anequation of the form

log [O₂ ]=γ−δ* pH

expression in which the coefficients α, β, γ, and δ are determined caseby case for each type of water.

In order to evaluate the treatment efficiency, the piping 7 for treatedwater, downstream from the biofilter 5, can be provided with asensor-analyser unit 21 measuring the oxidation-reduction potential Ehof the treated water and a sensor-analyser assembly 22 measuring itsresidual iron content. These assemblies are connected to a computer 23which can set off an alarm 24 if there is an anomaly. This embodiment isgiven as a non-limiting example. The different computers, 10, 14, 23,mentioned above can form a single component capable of integrating thedifferent signals emitted by the sensors 8, 9, 12, 13, 21, 22, and ableto command a single air flow regulator. This component can also comprisethe pH regulator 17.

In the above, the application of the invention as described in FIG. 1referred first and foremost to a biological iron removal treatment. Theinvention can regulate any other biological treatment based on oxidationby air, in particular biological manganese removal. The algorithms arethen less complex, since it is sufficient to ensure that the fundamentalphysico-chemical parameters of the process (potential, dissolved oxygen,possibly rH) are all well located above a certain set point, without itbeing necessary to take into consideration an upper limiting value,whatever the parameter under consideration.

In the same way, in another variant shown in FIG. 2, it is possible toput a device for iron removal according to the invention in series witha downstream device for manganese removal. The exit 7 from theferro-bacteria biofilter reactor 5 is linked directly or indirectly tothe entrance to a second aeration chamber 4′ into which the treatedwater from the ferro-bacteria biofilter reactor arrives together withthe air 2 injected by a second valve 3′. The water is then treated bypercolation through a second biofilter reactor 5′ provided with an exit7′ and comprising a porous mangano-bacteria supporting bed 6′. Thirdmeasurement means 9′ of Eh oxidation-reduction potential or dissolvedoxygen and measurement means 8′ of pH are set at the exit from thesecond aeration chamber. The representative measurement signals are sentto a computer 10′ which compares the representative signal of thepotential measured with a lower limit given as a function of therepresentative signal of the pH measurement. If the measured potentialis lower than the lower limit, the computer 10′ delivers a commandsignal to a regulation means 11′ acting on the second valve 3′ to enableadjustment of the air flow. It is to be noted that the computer 10′ canuse the pH measurement of the water treated by the iron removal device,made by the sensor 8 or the second measurement means 13 described above.The water exiting from the ferro-bacteria biofilter reactor can ifnecessary undergo special treatments before being treated by themanganese removal device.

The invention has been applied as a pilot station to the treatment andelimination of iron and manganese in ground water. The installationcomprises two filtration steps operating according to the processaccording to the invention, one adjusted for iron removal and the secondadjusted for manganese removal. The results are given in the tablebelow, and demonstrate the low concentration of iron and manganese inwater treated by the process according to the invention.

PARAMETERS RAW WATER TREATED WATER pH 7.0 7.8 Fe (mg/l) 13 <0.1 Mn(mg/l) 2 <0.04

FIG. 3 shows the evolution of the concentration in ferrous iron (CFe²⁺)of water treated in different plants as a function of time (Tps) oftreatment in hours. The curves (□, ◯, Δ) represent the results obtainedon an existing iron removal plant, operating entirely on thephysico-chemical principle of chlorine oxidation, followed by filtrationover manganese greensand. The curves (□, □, □) showing the resultsobtained on a pilot plant for biological iron removal, functioningaccording to the invention, were tested in parallel. The results, shownfor three filtration cycles speak for themselves and emphasise all theinterest of this type of biological treatment which demonstrates aconsistent low residual concentration of iron whereas for the existingphysico-chemical process, this rises with the hours of utilisation ofthe plant.

Also, the process according to the invention, on a biological manganeseremoval filter, was tested on a plant operating at a filtration speed of30 m/hr and the table below summarises the excellent results obtained.

PARAMETERS RAW WATER TREATED WATER pH 7.65 7.65 Mn (mg/l) 0.7 <0.02

Other modifications known to those skilled in the art are also withinthe scope of the invention.

What is claimed is:
 1. Method for elimination by biological route ofmetallic elements present in the ionized state in water devoid ofdissolved oxygen, in which the water to be treated is partiallyoxygenated by a specific aeration carried out before percolation througha biofilter reactor with a bacteria-supporting bed of filteringmaterial, characterised in that it comprises: a measurement stage for atleast one parameter constituted by the oxidation-reduction potential(Eh) of the aerated water before passage into the biofilter; atransmission stage of measurement signals to a computer and comparisonwith the signal representative of the value of at least one parametermeasured for at least one lower limit of this parameter determined infunction of the measurement carried out in a second stage of measurementof a second parameter representative of the pH and a possible stage forcorrection of the air flow by control by a unit (3, 10, 11) foradjusting the air flow by a signal determined by the computer infunction of the two preceding stages.
 2. Method according to claim 1,characterised in that it further comprises a stage for measuring thesecond parameter constituted by the pH of the aerated water beforepassage into the biofilter, and a stage of comparison with a lower andupper limit of the first parameter whose lower and upper limits aredetermined in function of the measurement of the second parameter. 3.Method according to claim 1, characterised in that it comprises a stagefor compensation of a fault in the adjustment of the air flow by thefirst parameter by means of a system of complementary regulation usingat least one signal provided by measurement means (12) of the residualcontent of oxygen dissolved in the treated water.
 4. Method according toclaim 1, characterised in that the compensation stage uses a secondsignal provided by measurement (13) of the pH of the treated watersimultaneously with that of the dissolved oxygen.
 5. Method according toclaim 2, characterised in that it comprises an adjustment stage of thepH of the filtered water through injection of an alkaline solution (19)into the water to be treated, if the signal provided by the measurementmeans (13) of the pH representative of the pH value of the treated wateris lower than an assigned lower value, said injection being limited byan assigned set upper value for the pH.
 6. Method according to claim 1,characterised in that it comprises a stage for verifying the efficiencyof the treatment by continuous measurement of the content of residualdissolved iron and the oxidation-reduction potential of the filteredwater, with an alarm being set off in the event of anomaly. 7.Utilisation of the method according to claim 2, characterised in thatthe method is used in a biofilter with a ferro-bacteria supporting bedof filtering material.
 8. Utilisation of the method according to claim1, characterised in that the method is used in a biofilter with amangano-bacteria supporting bed of filtering material.
 9. Utilisation ofthe method according to claim 1, characterised in that the method isused in a biofilter with an autotrophic—bacteria supporting bed offiltering material.
 10. Utilisation of the method according to claim 2,characterised in that the method is used in a first biofilter with aferro-bacteria supporting bed of filtering material, and then thetreated water exiting from this first biofilter is used according to themethod of claim 1 in a second biofilter with a mangano-bacteriasupporting bed of filtering material.
 11. Device for treating watersdevoid of dissolved oxygen, characterised in that it comprises anaeration chamber (4) into which the raw water and injected air arebrought, the flow of the latter being controlled by a valve (3) allowingcontrolled aeration under pressure, and whose exit is linked to abiofilter reactor provided with an outlet (7), and with a porousferro-bacteria supporting bed through which the water to be treatedpercolates, first measurement means (8) of the pH and second measurementmeans (9) of the oxidation-reduction potential (Eh) of the aeratedwater, set between the aeration chamber and the filter, the calculatingunits (10) taking into account the signals delivered by the first andsecond measurement means to send a command signal to a means forregulation of the air flow (11, 16) acting on the valve (3) to allowregulation in function of a lower limit and an upper limit for thepotential Eh, determined for a given pH value.
 12. Device according toclaim 11, characterised in that it comprises means (13) for measuringthe pH and means (12) for measuring the dissolved oxygen at the outletfrom the filter (5) and units for computing (14) and regulating (16) theair flow to allow complementary adjustment of the method.
 13. Deviceaccording to claim 11, characterised in that it comprises a station forregulating the pH comprising a reservoir (20) of an alkaline solution(19) controlled by an electro-valve or feed pump which is monitored bythe signal worked out by a regulation unit (17) and allowing theadjustment of the pH in function of the signal delivered by themeasurement means (13) of the pH set at the outlet from the biofilterreactor to the regulation unit (17).
 14. Device according to claim 11,characterised in that it comprises means (21, 22) for measuring theoxidation-reduction potential of the residual iron in the filteredwater, set at the outlet from the filter and allowing evaluation of theefficiency of the device.
 15. Device for treating waters devoid ofdissolved oxygen according to claim 11, characterised in that the outletfrom the ferro-bacteria biofilter reactor is linked to a second aerationchamber (4′) into which is brought the water treated ny theferro-bacteria biofilter reactor and air injected by a second valve(3′), the exit from the aeration chamber (4′) being linked to a secondbiofilter reactor (5′) provided with an outlet (7′) and having a porousmangano-bacteria supporting bed through which the treated water from theferro-bacteria biofilter percolates and third measurement means (9′) ofthe oxidation-reduction potential (Eh) at the exit from the secondaeration chamber, a calculation unit (10′) taking into account thesignal delivered by the third measurement means to send a command signalto a regulation means (11′) acting on the second valve (3′) to allowadjustment of the air flow in function of a given lower limit. 16.Device according to claim 11, characterised in that the filtering bed isconstituted of siliceous sand of an effective size comprised between 1and 3 mm.
 17. Device according to claim 11, characterised in that thefiltering bed is constituted of a filtering material, called “Biolite”.